Polymer composites comprising a polymer matrix having one or more additives such as a particulate or fiber material dispersed throughout the continuous polymer matrix are well known. The additive is often included to enhance one or more properties of the polymer. Useful additives include inorganic layered materials such as talc, clays and mica of micron size or glass fibers of various lengths. Generally, the addition of a filler enhances some properties, such as stiffness or tensile strength, at the expense of other properties, such as elongation or impact strength. In addition, fiber type reinforcements tend to improve impact strength while particulate type additives tend to have the opposite effect (i.e., lower impact strength). Furthermore, in either case, the exact influence of an additive on a given property may be a function of filler level and specific filler-resin interactions.
Nanocomposites are a new class of materials revealing characteristics, relative to traditional fillers, of significantly improving mechanical properties. Nanocomposites typically use relatively small amounts, usually less than 10%, of nanometer-sized particles to reinforce a polymer. They reportedly offer significantly enhanced mechanical, thermo-mechanical and barrier properties when the nanometer-sized particles are properly treated and dispersed into the polymer.
A number of techniques have been described for dispersing inorganic layered materials into a polymer matrix. It has been suggested to disperse individual layers, e.g., platelets, of the layered inorganic material, throughout the polymer. However, without some additional treatment, the polymer will not sufficiently infiltrate into the space between the layers of the additive and the layers of the inorganic material will not be sufficiently uniformly dispersed in the polymer.
To provide a more uniform dispersion, as described in U.S. Pat. No. 4,889,895, sodium or potassium ions normally present in natural forms of mica-type silicates and other multilayered particulate materials are exchanged with organic cations (e.g., alkylammonium ions or suitably functionalized organosilanes) thereby intercalating the individual layers of the multilayered materials, generally by ionic exchange of the sodium or potassium ions. This intercalation can render the normally hydrophilic mica-type silicates organophilic and expand the interlayer distance. Subsequently, the layered material (conventionally referred to as “nanofiller”) is mixed with a monomer and/or oligomer of the polymer and the monomer or oligomer is polymerized. The intercalated silicate is described as having a layer thickness of 7 to 12 angstroms and an interlayer distance of about 20 angstroms.
In WO 93/111900, an alternative method of forming a composite is described in which an intercalated layered particulate material having reactive organosilane compounds is dispersed in a thermoplastic polymer or vulcanizable rubber. Furthermore, additional composites containing these so-called nanofillers and/or their methods of preparation are described in U.S. Pat. Nos. 4,739,007; 4,618,528; 4,528,235; 4,874,728; 4,889,885; 4,810,734; 4,889,885; 4,810,734; and 5,385,776 German Patent 3808623; Japanese Patent J02208358; European Patent Applications 0398551; 0358415; 0352042; and 0398551 and J. Inclusion Phenomena 5,473 (1987); Clay Minerals, 23, (1988), 27; Polym. Preprints, 32 (April 1991), 65-66; Polym. Prints, 28, (August 19987), 447-448; and Japan Kokai 76,109,998.
The principle of utilizing a layered clay to enhance or improve the properties of a polymer matrix in which the clay has been dispersed has also been reported. U.S. Pat. No. 4,739,007 describes the use of a composite material comprising a polyamide matrix and well-dispersed silicate layers exhibiting high mechanical strength and excellent high temperature properties. As reported therein, it was believed that the clay particles in a nylon nanocomposite induce crystallization. It was also proposed that the polymer/clay nanocomposite upon processing leads to crystallization of the matrix polymer around the dispersed particles.
Nanofillers are also available based on tiny platelets of a special type of surface modified clay called montmorillonite. The two manufacturers in the United States, Nanocor and Southern Clay Products, both point to increases in flexural modulus, heat distortion temperature and barrier properties with the addition of such filler in selected polymers. Montmorillonite clays are reportedly hydrophilic in nature and are a naturally occurring raw material found in abundance in the USA. They are generally found in Wyoming, Montana and the Dakotas. The clays are mined and then processed into commercially available end products. The typical process control factors in the refining of the clay are:                Solid/water ratio        Counter ion optimization        Purity        Pre-organic reaction particle size        Organic/inorganic ratio        Post-organic reaction dispersive characteristic        Post-milling solids/Moisture ratio        Post-milling dispersive characteristics        Post-milling particle size        
The various organic compounds that are typically used to treat this clay are ammonium cations. These cations are used to minimize the attractive forces between the agglomerated clay platelets. The typical agglomerated platelets are separated by a distance of approximately 3.5 angstroms. The cation treatment presumably acts on the platelets to separate them. The separation distance will depend on the cation molecule used. Typically a distance of about 20 angstroms can be achieved. This intercalation process opens the spacing between the platelets enough so that the monomer or polymer can penetrate between the platelet layers. Without this, the reinforcing nature of the platelets will not occur.
The final step in making the polymer nanocomposite is, not surprisingly, either the process of combining the clay and polymer in a reactor in situ or by melt compounding the clay into the polymer using an extruder. In either case, sufficient shearing action in the reactor or twin screw extruder will determine the extent of exfoliation and dispersion of the nanoclay.
Turning next to a general discussion of catheters, for which the present invention finds particular application, it is noted that catheter designers are constantly faced with the challenge of making smaller diameter, thin walled catheters with increasing performance requirements. These catheters often need to have varying mechanical properties along their length to allow for manipulation of the device from a location several feet away from where the “action” is. For example, in a diagnostic electrophysiology catheter (see FIG. 1), the catheter shaft 10 needs to be flexible at the distal tip 12 to provide for deflection of the tip within the heart. The section in the middle of the shaft 14 needs to be somewhat stiffer in order to provide enough column stiffness to push the deflectable tip through the tortuous anatomy. The most proximal section 16 needs to be somewhat stiffer still to provide for maneuverability (pushability and torqueability) of the device from a distance of about a meter away from the catheter tip.
Polyurethanes, nylons, polyether block amide copolymers, and polyester elastomers are among the materials commonly used in the manufacture of such catheters. Furthermore, in order to achieve varying properties along the length of the catheter, segments of differing materials are usually welded, bonded, or intermittently extruded in a single catheter shaft. The ability to join these differing materials is a primary concern in selecting the materials for catheter shaft design. It is important that the adjacent materials be compatible for joining by the manufacturing process used and not detach during medical use. However, joining together differing materials can cause added thickness and rigidity at the joining points. In addition, fusion bonding can cause crystallization at the bonding points which may add stiffness.
Historically, material choices (whether for a catheter or any other application) have been somewhat limited by the offerings of material suppliers. For example, when evaluating the flexural modulus between grades of polyether block amide copolymer, it is not uncommon to find that the flexural modulus approximately doubles for each sequential commercial grade offered. Therefore, in order to achieve an intermediate stiffness, designers have been forced to blend different grades of these materials or choose materials from a different polymer family in order to secure the balance of properties required by their specific catheter designs. This in turn can have a negative impact on processability of the materials during manufacturing and often results in a net reduction on the overall balance of the physical, thermal and barrier properties of the final blended polymer.
U.S. Pat. No. 5,584,821 discloses an angiographic catheter which has a relatively stiff though flexible shaft and a soft tip. The soft tip consists primarily of a tungsten loaded polyether block amide (PEBA) copolymer surrounded by two thin PEBA layers. This three ply radiopaque tip is bonded to a PEBA shaft. The shaft is reinforced either by an inner nylon ply or by metal braiding.
Similarly, when it comes to the production of a soft tip catheter with a relatively stiffer body, U.S. Pat. No. 5,584,821 emphasizes that the stiffer body portion relies upon the use of a metal braided reinforced PEBA copolymer or a co-extruded two ply wall consisting of nylon and PEBA copolymer. That being the case, it becomes clear that inasmuch as PEBA type copolymers are widely used in catheter type applications, it would serve a long-standing need if one could conveniently produce a more rigid and toughened PEBA catheter, without the need for the structural modifications emphasized in the prior art.
Thus, there remains a need in the art for a single polymer that can offer a balance of properties such that only the filler level may need to be varied to provide a catheter which is contiguous and not liable to detach or come apart during use.
Use of copolymers such as nylon block copolymers containing polyamide segments and elastomeric segments has been reported in U.S. Pat. No. 4,031,164. Cross-linked nylon block copolymers are described in U.S. Pat. Nos. 4,671,355 and 5,198,551. Radiation cross-linked nylon block copolymers are described in U.S. Pat. Nos. 5,993,414 and 5,998,551 and in U.S. application Ser. No. 10/129,921 which are all assigned to the assignee of the present invention and included herein by reference.
Accordingly, it is an object of this invention to provide a nylon/nanoclay composition which has a unique synergy in properties and which can provide particular utility as one or more components of a medical catheter product.
More specifically is an object of the present invention to provide a nylon 12/nanoclay composition wherein the composition exhibits improvements in properties such as flexural modulus, tensile modulus and impact strength while maintaining ductility and elongation even at relatively high nanoclay loadings. Thus, a balance of flexibility and stiffness would be achieved which makes the composition particularly useful as a catheter or similar surgical instrument due to the unique balance of properties obtained.
These and other objects of the present invention, together with the advantages thereof over the prior art, which shall become apparent from the specification that follows, are accomplished by the invention as hereinafter described and claimed.